Inspired by the work of Jean-Claude Risset who created a version of the musical scale where the steps between each tone are continuous, I have decided to create filter based on the same principles for use in my sound design/production works.

For those unfamiliar with “Risset Tones” they form a subjectively continuous glissando and are based on the work of cognitive scientist “Roger Shepard’s” "Shepard Scale". In essence; the Risset/Shepard Tones/Scales collectively form an auditory illusion composed out of a series of ascending or descending tones. When done correctly, the tone appears to rise (or descend) continuously in pitch, yet return to its starting note. Risset has also created a similar effect with rhythm in which tempo seems to increase or decrease endlessly.

This is similar in regards to the Visual Illusion known as the Penrose Staircase, although an auditory illusion in this case.

The Program

My concept was to use the same “glissando” as Risset, however, rather than playing sine waves at the centre frequencies as Risset devised, I created a bank of bandpass filters who’s cut-off frequencies are centred at those frequencies and move up/down in tandem.

Although I would have liked to document the entire coding process and the logic behind such it really would take me a week to do so, instead I will just post the results.

There were two major revisions of the Risset filter program:

Version One

was created so that the output was MIDI data, in this way I could use the data to control other MIDI capable devices, in the example case I use a standard EQ Eight VST from ableton live.

Version Two

Was recreated with Max4Live and is a standalone audio processor with the filter banks built into the device.